There have been two outbreaks of human poisoning by polychlorinated biphenyls (PCBs) and their thermal breakdown products; the first, in Japan in 1968, the second in Taiwan in 1979. Because PCBs are a world wide pollution problem, these episodes have been studied carefully, since they have presented the only opportunity to observe directly the toxicity of PCBs in human beings outside the workplace. Laboratory methods for the evaluation of these outbreaks were relatively unsophisticated in 1968; there has been great progress in analytical methods since. In collaboration with Taiwanese scientists, the Epidemiology Branch, NIEHS, had the opportunity to examine over 100 children who had been in utero at the time of 1979 poisoning or afterward. These children continued to be affected, since the chemicals cannot be excreted from the mother's body. We have examined blood and cerumen samples for 2,3,7,8- tetrachlorodibenzofuran and hexachlorodibenzofuran using selected ion monitoring at a mass resolution of 5,000. Instrument sensitivity during these analyses was such that 50 femtograms of analyte could be detected on an absolute level with a signal to noise of ca. 20:1. Taking into account sample volume and recovery yields, the practical limits of detection in the actual samples was ca. 2.4 picogram per sample (2.4 parts per trillion). No TCDFs or HCDFs were observed in these samples. To verify these results, selected samples will be analyzed using the selected decomposition monitoring capabilities of the concept I-SQ hybrid mass spectrometer. This technique is less sensitive than the high resolution selected ion monitoring technique but it is more selective. These results are also consistent with the PCB results obtained by RTI in which no PCBs were observed.